I don't know, but I think the first
three pādas of today's verse are a kind of exploration of its
causes.

The suggestion in the 1st
pāda is that the first cause of brightness is not to do wrong. Hence
King Śuddhodhana is described as puṇya-karmā,
which means a virtuous or pious man. At the same time, puṇya-karmā
might be intended to suggest a religious man, a maker of merit who is
concerned with keeping his karma pure for the purpose of receiving a
future reward, for example, in heaven.

The 2nd pāda, as I read it,
is therefore designed to subvert this namby-pamby conception with two
much more materialistic expressions of the causes of brightness. Real
brightness, the 2nd pāda suggests, is not something
spiritual but something arising from power, energy, intense effort.

The 3rd pāda says to both previous pādas, who might be seen as squabbling with each
other, “No, it is not that.” In a practical spirit of reason,
compromise and synthesis, the 3rd pāda says that both
nature and nuture (represented by kula, family) are determinants, but so
also is a person's own autonomous action (vṛtta) important, and in the middle
way between determinism and freewill a factor like good sense (dhī) might be difficult to put on one side or the other.

In comparing the King to the
thousand-rayed sun (sahasrāṁśuḥ), Aśvaghoṣa reinforces the
impression that in praising the virtues of the Buddha's father, who
by definition pre-dated the Buddha, his real intention is to
describe the virtues of Gautama Buddha as universal, or non-Buddhist.
Those real virtues of Buddha are as universal, as irreligious, and as
non-Buddhist as, for example, the 2nd law of
thermodynamics.

Dogen said that mountains belong to
people who love mountains. By the same logic, the sun might have
belonged in antiquity to the Aztecs or to the ancient Japanese –
but not to one group at the expense of the other. I think partly
because the virtues of the sun are not Buddhist but universal,
Aśvaghoṣa uses the sun three times in Saundarananda as a symbol of
the Buddha:

To people possessed by ends, serving
many and various paths, / Splendour had arisen that seemed like the
sun: Gautama was like the sun, dispelling darkness. // SN3.16 //

He walked over water as if on dry land,
immersed himself in the soil as though it were water, / Rained as a
cloud in the sky, and shone like the newly-risen sun. // SN3.23 //

One woman there, however, on glancing
through a round side-window on the upper storey of the palace, / Had
seen the Sugata, the One Gone Well, going away -- like the blazing
sun emerging from a cloud. // SN4.28 //

So ostensibly the 4th pāda
describes King Śuddhodhana as bursting with fiery energy, or as
desiring to shine forth, but below the surface it might be intended
to suggest the desire of the King's son, Gautama Buddha. And on a
still deeper level, whether Aśvaghoṣa intended it or not, I read
the 4th pāda as an autobiographical expression of
Aśvaghoṣa himself, whose crafting of poetry like this should not
be denigrated as a work of religion or literature, but should rather
be appreciated as brightness emitted by somebody ineffable, who
practised the backward step of turning his light and letting it
shine.

karman:
n. act, action ; any religious act or rite (as sacrifice , oblation
&c , esp. as originating in the hope of future recompense and as
opposed to speculative religion or knowledge of spirit); former act
as leading to inevitable results , fate (as the certain consequence
of acts in a previous life)